The invention relates to a method and apparatus for noninvasive assessment of cardiac performance by detection of compression waves generated by heart muscle function. The method and apparatus involves the detection of cardiovascular abnormalities for monitoring the function of the heart muscle by detecting at the exterior surface of the body, compression waves generated by heart muscle function.
Coronary artery disease is one of the leading causes of death and disability in the United States. Currently, there are an estimated six million individuals in the United States who have symptomatic coronary artery disease. Undoubtedly, there are many more with the disease who have not yet felt the symptoms of the disease due to its particular characteristics. Some authorities have estimated that about half of those affected with the disease are in apparently good physical health and are totally unaware of the presence of the disease. In 1985 an estimated 540,000 deaths occurred due to coronary artery disease and more than 700,000 patients in the United States were hospitalized with acute myocardial infarction. Accordingly, researchers and physicians who specialize in this area are constantly striving to develop simple and safe methods for early detection of the diseases of the heart. The objective is to develop methods and apparatus which are cost-effective so that they can be used on a routine basis.
Because of the potentially large number of apparently healthy people who have various stages of cardiovascular disease, a number of attempts at detecting the disease have been tried. Currently, there are several methods used for testing for myocardial ischemia. One such test is coronary angiography. This test is a reliable method for detecting narrowing of coronary arteries. This invasive procedure requires the insertion of catheters into or near the heart. Because of the invasive nature of this test, it involves risk of medical complications. Occasional fatalities will result from the procedure. Because of the risks of using this method and because of the cost attendant to use of the technique, physicians and researchers are constantly attempting to develop a variety of noninvasive techniques which can achieve reliability in providing accurate information concerning the extent and nature of cardiovascular disease.
The most widely used technique for noninvasive screening of patients for coronary artery disease involves electrocardiography. This noninvasive method and the apparatus utilized for conducting the tests detects the bioelectric potential of the heart muscle and provides a graphic display of the bioelectrical activity in the muscle for analysis by the physician. The display may be used for later comparison with additional electrocardiograms.
Exercise electrocardiography (ECG) is frequently used in a technique which involves making two or more records of the patient's heart muscle electrical function. In this technique, the individual's "at rest" cardiovascular function is first recorded. Then the patient undergoes a period of controlled strenuous exercise. During the course of this controlled strenuous exercise, additional electrocardiograms are prepared for comparison to the ECG which has been recorded at the patient's "at rest" condition. The comparison is made on the theory that strenuous exercise will produce a transient imbalance in the patient's heart oxygen supply. It is believed that this transient imbalance occurs between the heart's oxygen requirements and the oxygen which is supplied by the blood flow in the coronary artery. It is believed that this oxygen imbalance will be imposed by the strenuous exercise. Such an imbalance may be detected by the development of electrocardiographic abnormalities which are believed typical of coronary artery disease.
Unfortunately, it has been learned that the electrocardiographic technique seems to be relatively insensitive to the oxygen imbalance that apparently occurs, and therefore, the reliability of this technique is not as high as might be desired. Some authorities have placed the probability of successful diagnosis using this technique at only about sixty percent (60%) in patients with symptoms suggestive of coronary artery disease. Further, it has been found that as many as half of the patients who exhibit abnormal ECG readings during the strenuous exercise portion of the test, but who are otherwise healthy, do not actually have coronary arterial disease. Therefore, while exercise electrocardiography provides a relatively safe, noninvasive technique which is relatively low in cost, it, nevertheless, provides a procedure which does not have the accuracy desired by the medical profession.
Other examples of alternate procedures to coronary angiography are such noninvasive techniques as exercise radionuclide ventriculography, exercise thallium-201testing and exercise echocardiography.
Exercise radionuclide ventriculography (MUGA) is a procedure of noninvasive testing which requires the injection of radioactive materials. Myocardial function deteriorates rapidly when ischemia develops. In animal models, a heart wall motion abnormality develops in the heart muscle within seconds of coronary artery occlusion in that part of the myocardium perfused by the occluded artery. Exercise radionuclide ventriculography takes advantage of this condition. The test is done by radioactive labeling of the patient's red blood cells, so that scans of the heart show the internal shape and amount of contraction. Images obtained before exercise can then be compared to exercise and post-exercise scans. This procedure is expensive and time consuming. While the sensitivity of this method is relatively good, its specificity, and, therefore, its predictive accuracy is somewhat less than desired.
Another method of noninvasive testing involves the use of thallium-201, which is a radioactive material that can detect blood flow abnormalities to the heart during myocardial ischemia. Thallium-201 is an analogue of potassium, the major intracellular cation of the heart muscle. The thallium therefore quickly enters functioning heart muscle cells, but does not enter ischemic cells. When injected during exercise, the thallium image will show poor uptake in areas of ischemia. This test also is time consuming as well as relatively costly. The method requires the injection of radioactive material and, for this and other reasons, physicians continue to desire less stressful and less time consuming methods of examining and testing patients.
A third technique, exercise echocardiography, attempts to visualize cardiac wall motion abnormalities induced by ischemia by obtaining ultrasound images of the heart before, during and after exercise. A handheld probe applied to the chest wall transmits and receives ultrasonic waves directed at the heart. Problems with the o technique include the need for considerable expertise in obtaining the images, frequent failure to obtain adequate images during exercise because of motion artifact and high equipment costs.
As mentioned above, tests on animal subjects have shown that reduction in blood supply to the heart muscle can be detected by studying the movements of segments of the heart wall. These studies have revealed that when segments of the heart muscle receive less oxygen than required for proper function, the normal contraction pattern of the heart muscle will change. The change in heart muscle function occurs because of the lack of oxygen supplied to the muscle through the coronary arteries. It is these abnormalities in the function of the heart muscle itself which are of concern to the physician. Other methods of noninvasive detection of heart muscle function have limitations which are troublesome when the methods are used for screening patients. Methods using the electrical field (e.g. electrocardiography) effect around muscle tissue are presented with problems of shielding the environment in which the patient is being tested from extraneous electrical fields. Testing the muscle function of the heart by measuring the electrical field at or in the heart area requires detecting changes in the electrical field that are extremely small. These small changes in the electrical field can be affected by other diagnostic or electrical equipment which is frequently operated in the immediate area of the testing site.
Similar problems can occur when testing systems rely on changes in the electromagnetic field surrounding the heart (e.g. cardiokymography). Detection of changes in the electromagnetic field in or surrounding the heart also requires detection of extremely small changes in an electromagnetic field. Again, the detection of relative weak magnetic fields can be dramatically affected by equipment operated in the same room or in the proximity of the test site. These uncontrolled fields, of course, present a serious problem for these testing methods since they can obscure or destroy the data which is collected.
The present invention avoids the use of extremely weak electric and electromagnetic fields and relies on wave energy of the type analogous to seismic waves generated in the crust of the earth as a result of earthquake activity. The functioning of the heart muscle generates compression waves which are transmitted throughout the body just as seismic waves travel throughout the earth when an earthquake occurs. In the case of the human heart, the natural muscle function involved with the contraction and relaxation of the muscles of the heart generate compression waves which have been found to travel throughout the body of the patient.
Experimental investigation has revealed that the nature of these compression waves generated by the heartbeat infer detailed characteristics of the specific muscle function of the heart and can be relied on for detecting abnormal muscle function. As previously indicated, when oxygen supply to the muscle of the heart is interrupted, the muscle reacts accordingly. The muscle may cease contractions or may produce contractions that are weak or in some way altered by the lack or reduction of oxygen supply to the muscle. These abnormal muscle contractions or function produce compression waves which are indicative of the abnormal muscle function. Detection of this abnormal muscle function then can indicate the presence of coronary arterial disease in a noninvasive fashion so that proper treatment can be applied to the disease.
The present invention provides an effective method of noninvasive testing and screening patients for coronary arterial disease since the detection of the compression waves generated by heart can be detected at the outer surface of the body. The procedure does not require injection of radioactive substances, chemicals, or catheters for treatment. It can be administered quickly by properly trained medical personnel. Further, it provides for a graphic record for comparison with normals and for baseline recording to compare to future recordings in the same individual, and for future analysis or comparison with further such tests.